The invention relates generally to LC circuits and, more specifically, to a method and apparatus for damping of an LC circuit between a power converter and an electric grid.
Renewable power generation systems include power sources such as solar panels or wind turbines, for example, and power converters for converting power from the power sources into power having an alternating current with controlled amplitude and frequency. It is desirable that the alternating current supplied to an electric grid be synchronous with the grid voltage or current and that harmonic components are minimized. Some power systems use LC filters to minimize high frequency harmonics.
A typical LC filter includes inductors and capacitors. LC filter resonance is defined as the condition when the inductive reactance and capacitive reactance of the LC filter are of equal magnitude. The frequency at which resonance occurs is defined as the resonant frequency. Mathematically, the resonance frequency may be expressed as:
  fr  =      1          2      ⁢      π      ⁢              LC            wherein “L” is the equivalent inductance, “C” is the equivalent capacitance, and “fr” is the resonant frequency. At the resonant frequency fr, the impedance of the filter is minimal. Accordingly, a resonance peak may be excited by the high frequency harmonics and delivered to the grid. To attenuate such a resonance peak, a damper for the LC filter may be used.
One conventional method of damping an LC filter is to couple a physical resistor in series or in parallel with the capacitor of the LC filter. Drawbacks to the resistor embodiment are that an undesirably large amount of power is consumed by the resistor and that the LC filter becomes more bulky because the resistor occupies significant space.
Ma et al., U.S. Pat. No. 6,166,929, described a method of damping an LC filter between a three-phase inverter and a motor by simulating a damping resistor connected in parallel with each output capacitor of the LC filter. The simulation included a determination of how much current would flow through a resistor had it been there and deducting that amount of current from a current command that is used to control the inverter. The damping method described by Ma et al. is very specific to motors and has some of the benefits of a physical damping resistor without the corresponding energy loss.
As compared with motors, when providing power from renewable power sources to an electric grid, there are more variations and transient events. In addition, simulating a physical damping resistor for damping an LC filter in the inverter control may adversely affect stability of power conversion control.
There is a need in the art to provide an improved method and system for damping an LC circuit between a power source and an electric grid which differs from conventional methods or systems and addresses one or more of the above discussed problems.